Oh dear! I can see lots of new threads in disputes and prosecutions over this.
Yes, the typical scenario of someone who has touched in on Oyster at St Pancras and got to the Thameslink barrier at, say, Harpenden to be told they can't touch out. "But there is a touch out point there!" they say pointing at the barrier. "Ah, that's for a different system sir." the staff tell the passenger. "But I used Oyster to go one stop further to Luton Airport last week!"' "Well, if you would like to continue to Luton and touch out there you can avoid a penalty fare and then buy a single to come back here."
I was in Seoul last week which has a huge transport system. Their Oyster equivalent is valid on metro, buses, taxis, purchases in convenience stores, transport scheme in other cities.... one day.